In the oil and gas industry, geophysical prospecting is used to aid in the search for and evaluation of subterranean formations. Geophysical prospecting techniques yield knowledge of the surface and subsurface structure of the earth, which is useful for finding and extracting valuable mineral resources, particularly hydrocarbon deposits such as oil and natural gas. One technique associated with geophysical prospecting is a seismic survey. In a marine seismic survey, a seismic signal is generated in a body of water, such as an ocean. Seismic energy sources such as water guns, air guns, and explosives are used to generate the seismic signal. Marine seismic surveys typically employ a submerged seismic source towed by a ship and periodically activated to generate an acoustic wavefield. The acoustic wavefield may be a complex wavefield that includes many different frequencies. In many cases, the seismic source consists not of a single source element, but of a spatially-distributed array of source elements.
The signal travels from the source down through the water (water layer) to the earth and possibly through sub-surfaces of the earth, where the signal is at least partially reflected by surface or subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, such as rock density, which lead to differences in acoustic impedance. Different subsurface layers may also have different attenuation and dispersion effects on the acoustic wavefield. The seismic energy is reflected back up through the water by the seismic reflectors, and the reflected seismic energy is detected and recorded by seismic sensors (also called seismic receivers, seismic recorders, or seismic detectors) such as particle velocity sensors (“geophones”), water pressure sensors (“hydrophones”), and acceleration sensors (“accelerometers”) in the water layer. Seismic sensors may be deployed by themselves, but are more commonly deployed in sensor arrays. Additionally, different types of sensors, such as hydrophones and geophones, may be deployed together, such as being collocated in pairs, in a seismic survey.
In an example marine seismic survey, a seismic survey vessel travels on the water surface, typically at about 5 knots, and contains seismic acquisition equipment, such as navigation control, seismic source control, seismic sensor control, and recording equipment. The seismic source control equipment causes a seismic source towed in the body of water by the seismic vessel to actuate at selected times. Seismic streamers, also called seismic cables, are elongate cable-like structures towed in the body of water by the seismic survey vessel that tows the seismic source or by another seismic survey ship. Typically, a plurality of seismic streamers are towed behind a seismic vessel. The seismic streamers contain sensors to detect the reflected wavefields initiated by the seismic source and reflected from reflecting interfaces. The hydrophones and geophones may be collocated in pairs along a seismic cable.
After the reflected wave reaches a streamer cable, the wave continues to propagate to the water/air interface at the water surface, from which the wave is reflected downwardly, and is again detected by the sensors in the streamer cable. The water surface is a good reflector and the reflection coefficient at the water surface is nearly unity in magnitude and is negative in sign for seismic signals. The waves reflected at the surface will thus be phase-shifted 180 degrees relative to the upwardly propagating waves. The downwardly propagating wave recorded by the receivers is commonly referred to as the surface reflection or the “ghost” signal. Because of the surface reflection, the water surface acts like a filter, creating spectral notches in the recorded signal and making it difficult to record data outside a selected bandwidth. Because of the influence of the surface reflection, some frequencies in the recorded signal are amplified and some frequencies are attenuated.
A vessel may tow very long streamers which have many seismic receivers attached. These receivers register a portion of a scattered acoustic wavefield originated from the sounding of a seismic source. The acoustic wavefield generated by the seismic source is scattered by subsurface reflectors and diffractors in the earth. In conventional marine seismic acquisition, the receivers of the streamer are located in array configurations at a particular depth position below the sea surface. Because of this arrangement, the so-called primary reflection, the direct response from source to subsurface and subsequently to the receivers, is clouded by the ghost reflection (signal noise), from the wave that travels from source to subsurface and subsequently via the sea surface to the receivers. The ghost reflection may be removed from marine seismic data to increase the resolving power. This procedure is known as “deghosting”.
The resulting seismic data obtained in performing the survey is processed, for example, by deghosting the seismic data, to yield information relating to the geologic structure and properties of the surface or sub-surface formations in the area being surveyed. For example, the processed seismic data may be processed for display and analysis of potential hydrocarbon content of subsurface formations. Seismic data may be processed and analyzed in order to image or otherwise characterize the geologic subsurface. Accurate characterizations of the geologic subsurface may facilitate geophysical prospecting for petroleum accumulations or other mineral deposits. In order to identify locations in the earth's subsurface where there is a probability for finding petroleum accumulations, large sums of money are expended in gathering, processing, and interpreting seismic data.
An image of the structure of the earth's subsurface may be used to select locations with the greatest probability of having petroleum accumulations. To verify the presence of petroleum, a well is drilled. Drilling wells to determine whether petroleum deposits are present or not, is an extremely expensive and time-consuming undertaking. For that reason, improvements in the processing and display of the seismic data may be quite valuable for assessing the probability that an accumulation of petroleum exists at a particular location in the earth's subsurface.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.